Dennis Massy-Dawson born in Madras, India on 6 December 1909, the son of Charles Godfrey Massy-Dawson and Wilhelmina Geraldine Massy-Dawson (née Haughton) of Lindfield, Sussex. He joined the Royal Navy as a Cadet and was promoted to Midshipman on 1 September 1928 and appointed to the 15-inch Monitor HMS EREBUS for training. He was promoted to Acting Sub Lieutenant on 1 January 1931 and, in July 1931 he was completing his Lieutenants Courses at RN College Greenwich to which he had been appointed on 1 January 1931.

He was appointed to HMS DOLPHIN for the Submarine Course on 9 May 1932 and was promoted to Lieutenant on 1 August 1932. After qualifying as a Submariner, he was, on 19 August 1932 appointed to the Submarine Depot Ship HMS MEDWAY for HMS PHOENIX as Third Hand (Fourth Submarine Flotilla, China Station) and, on 15 December 1934 he was re-appointed to HMS MEDWAY for PHOENIX as Navigating Officer.

He returned home on 30 March 1935 and appointed to the Submarine Depot Ship HMS TITANIA (Sixth Submarine Flotilla) at Portland for Submarine H34 as Navigating Officer. Dennis Massy Dawson returned to the China Station on 7 August 1936 when he was appointed to the Submarine Depot Ship HMS MEDWAY at Hong Kong initially for Submarine Duties and, subsequently for HMS REGULUS as First Lieutenant to date 14 April 1937.

Massy-Dawson was appointed to HMS DOLPHIN for the Commanding Officers Qualifying Course on 2 January 1939. He was then appointed to the Submarine Depot Ship HMS LUCIA (Second Submarine Flotilla) at Devonport for HMS SEAHORSE in Command on 15 April 1939. After the outbreak of WWII HMS SEAHORSE was transferred to the Submarine Depot Ship HMS TITANIA (Sixth Submarine Flotilla) at Blyth. In late December 1939 HMS SEAHORSE was sent on a patrol off the West Coast of Denmark and was then transferred to an area in the Heligoland Bight off the estuary of the river Elbe. On 7 January 1940, the German First Submarine Minesweeping Flotilla made an attack on a Submarine – believed to have been HMS SEAHORSE.

Dennis Massy-Dawson was married in Plymouth, Devon on 15 July 1939 to Jean Mary Hamilton-Campbell, who survived him.

Dennis Massy-Dawson is commemorated on the Portsmouth Naval War Memorial on Panel No. 37 column No. 1.